How to Pull "Sign In User ID" for Contact

Maybe someone can help me with the syntax for this report.  I think it should be possible using JoinFieldValue(), but no matter what I try, it causes a Syntax Error in the expression builder.

We need to be able to pull the "Sign In User ID" for a list of Contacts and their associated Accounts.  Am using the "Contacts" Subject Area in Reporting.

It seems I should be able to create a Custom Field containing the expression: JoinFieldValue('<User>', Employee."Employee ID", '<UserSignInId>').  No go.

To test the function, I even tried to pull the E-mail address of the Contact owner using:  JoinFieldValue('<User>',[<OwnerId>],'<EMailAddr>'), straight out of the Help function.  This doesn't work either.